Department of Physics, College of Science, Taiyuan University of Technology, Taiyuan 030024, China. nankailxc@yahoo.com.cn

Abstract

The ultraviolet light induced absorption change (UV-LIA) of nearly stoichiometric LiNbO3 : Fe : Mn crystals was investigated. The experimental results show that the UV-LIA coefficient change of LiNbO3 : Fe : Mn crystal is not large for congruent sample, increases with increasing Li2 O concentration, reaches the maximum 4. 2 cm(-1) at about 49.57 mol% Li2 O, and then decreases with further increasing Li2 O content. Because the UV-LIA change has a direct relationship with the nonvolatile holographic sensitivity, the experimental results indicate that the nearly stoichiometric LiNbO3 : Fe : Mn crystal with 49.57 mol% Li2 O is the appropriate candidate material for the nonvolatile holographic storage. The visible light induced bleaching results also prove that the suitable composition is 49.57 mol%. With the increase in Li2 O concentration in the LiNbO3 : Fe : Mn crystal, the amount of the bipolaron increases. Bipolarons may be dissociated either optically or thermally so that metastable small polarons are formed. The energy level for biopolaron and small polaron is at about 2.5 and 1.6 eV respectively. When the Li2 O concentration continues to increase, the small polarons are dominating intrinsic defects. The bipolarons have stronger photorefractive capability than the small polarons. The amount of bipolaron is the most with 49.57 mol% Li2 O concentration in the LiNbO3 : Fe : Mn crystal. Based on these experimental results, a three-photorefractive-centers model in nearly stoichimetric LiNbO3 : Fe : Mn crystal is suggested: besides Fe2+/Fe3+ and Mn2+ /Mn3+, bipolarons/small polarons are considered as the third photoactive center.